The present invention relates to the art of television camera systems and more particularly to a system for compensating for cable attenuation introduced as a result of arbitrary cable lengths being used to interconnect a camera head with a remote camera control unit.
Studio television camera systems presently available provide for control of a number of television camera heads from a single remote camera control station. This is necessary in order to provide uniformity of picture quality as well as to insure proper timing between the various video signals supplied by the respective camera heads. In these systems, each of the cameras is interconnected with the camera control unit via a transmission cable which carries the respective timing, control, and video information to and from the camera.
Since this transmission cable may be of arbitrary length, anywhere from several hundred yards to in excess of a mile, the amount of attenuation introduced as a result of the presence of this cable will vary extensively. In the past, these variations have been balanced out through the use of an operator-controlled adjustment which was readjusted whenever a new cable was connected between the camera head and the control unit. A multiconductor cable having a separate conductor for each control signal was generally used to communicate the various signals between the camera head and the camera control station. Although this approach allowed extreme simplicity in the manner in which signals were communicated back and forth, the cost of the cable became a prohibitive factor when large distances were to be covered between the camera head and the control unit.
Much work has thus been directed to the problem of combining the signals to form a composite signal which could be easily transmitted along a standard triaxial transmission line. When a color television camera is being used, this composite signal must include video information from the red, blue, and green video plumbicon tubes, as well as power supply, timing, and control signals. In addition, a black and white video signal may be returned to the viewfinder of the camera head from the camera control station along the same cable. These signals may be combined by frequency division multiplexing, however the composite signal will then be quite broadband and may extend over a frequency band in excess of 60-MHz. The use of a broadband composite signal introduces complications with regards to transmission cable attenuation. Since the attenuation provided by a triaxial transmission line increases with the frequency of the transmitted signal, attenuation of the higher frequency channels will be significantly greater than in lower frequency channels and may lead to unacceptable degradation of the signals carried therein when lengthy transmission lines are used. Cable loss compensation of some form is thus required in a system of this nature. Any compensation scheme must also take into consideration the fact that the attenuation will not be the same for different frequency channels.
The present invention provides a cable loss compensation system which not only provides automatic adjustment of the gain of the incoming signals in accordance with a detection of the amount of cable attenuation, but also provides different gains in different frequency channels so as to substantially equalize the gain of the signals in the various frequency bands.
In accordance with the present invention, an automatic cable loss compensation system is provided for a television camera system wherein a television camera communicates with a camera control unit by means of a broadband RF signal communicated over a transmission cable of arbitrary length. The compensation means includes an attenuation detector which is responsive to the signal in a selected frequency band of the broadband RF signal to provide a control signal having a value which is dependent upon whether or not the peak amplitude of the signal is greater or smaller than a preselected reference level. An attenuation pad is provided which is normlly connected in series with the transmission line to prevent ghosting resulting from RF signl reflections on the cable, but is bypassed whenever the control signal indicates that the signal strength has fallen below the preselected reference level.
In accordance with another aspect of the present invention, the attenuation detector also compares the peak amplitude of the signal with a second reference level to provide a second control signal having a state which depends upon whether or not the peak signal is above or below the second reference level. This second control signal is then used to either insert or bypass an additional stage of RF amplification in one or more of the higher frequency channels of the broadband signal.
In accordance with yet another aspect of the present invention, this second control signal also controls the insertion or bypassing of another attenuation pad at the output to a viewfinder transmitter which generates high frequency RF signals which are to be communicated to the camera head.